1. Field
Example embodiments relate to a light emitting device having improved light extraction efficiency, and more particularly, to an organic light emitting diode (OLED) including a light extraction layer for improving light extraction efficiency.
2. Description of the Related Art
A light emitting diode (LED) may be a point light source formed of compound semiconductor materials. Compared to LEDs, an OLED may have many advantages such as low power consumption, high outdoor visibility, and flexibility in addition to surface light emission. For this reason, the OLED may receive attention for applications as a lighting device in addition to a display device. However, the OLED may have low light extraction efficiency because only about 20 percent of generated light typically may be emitted to the outside. Such a low light extraction efficiency mainly results from the difference in refractive indices of an organic light emitting layer and external air. That is, among the light generated in the organic light emitting layer, only the light of a specific angular region is emitted to the outside and the light of a remaining angular region is absorbed in the inside of the OLED and disappears due to total reflection at the interface of air.
Light extraction layers having various structures may be used for improving light extraction efficiency of the OLED. For example, a light extraction layer having a micro-lens array shape may be attached on the external surface of a substrate. However, the external light extraction layer attached on the external surface of the substrate may not reduce (and/or prevent) optical losses occurring between internal layers of the OLED.
An inner light extraction layer may be disposed between a substrate and a transparent electrode. The internal light extraction layer may include uneven surfaces or scatters to change a light path to the outside; consequently, the surface flatness of the transparent electrode may be deteriorated. Once the surface flatness of the transparent electrode is deteriorated, charges such as holes and electrons may concentrate in a specific region and the electrical characteristics of the OLED may deteriorate. Therefore, in order to reduce (and/or prevent) the deterioration of the electrical characteristics of the OLED, a planarization layer may be further added between a light extraction layer and a transparent electrode.